Frequency hopping is a technique that may be used to enhance the frequency and the interference diversity in mobile telecommunications systems. For example, frequency hopping is employed in GSM and it is also an option in the uplink of the E-UTRA (Enhanced UMTS Radio Access) also referred to as 3GPP Long Term Evolution, LTE.
Frequency hopping spreads the transmission over several different frequencies according to a certain pattern. This pattern must be known by both the transmitter and the receiver. Information regarding the frequency hopping pattern may be broadcasted or signaled in another way. This provides frequency diversity given that the different frequencies are well separated. The separation should exceed the channel coherence bandwidth. Moreover, interference diversity is achieved if different hopping sequences are employed by different users. Frequency hopping should, preferably, be performed within a channel coded block such that the enhanced diversity may be taken advantage of channel coding and interleaving.
FIG. 1 gives an example of a possible frame structure comprising eight bursts. The respective bursts of a channel coded packet having the frame structure of FIG. 1 are transmitted on the frequencies f0, f1, . . . , f7. The duration over which the channel coded packet is transmitted is often called the transmission time interval (TTI) here corresponding to eight bursts.
A disadvantage with frequency hopping is that it may increase the required reference signal overhead for a given demodulation performance. With frequency hopping, reference signals for demodulation must be provided for all used frequencies. For example, for the frame structure in FIG. 1, wherein 8 bursts 101 are transmitted in one TTI (Transmit time interval) 100 reference signals for all used frequencies f0, f1, . . . , f7 must be available when the data is demodulated. The reference signals may comprise known symbols that are received in order to learn how to demodulate unknown symbols. E.g. a “0” and a “1” may be transmitted as reference signals and the receiver can then learn how a received “0” and “1” looks on a particular channel and the receiver may then demodulate unknown symbols based on the reception of the reference signal.
In GSM, reference signals (also referred to as pilots, or training sequences) are included in each burst. I.e., each burst is self-contained. In E-UTRA uplink, reference signals are separated from the data and transmitted in separate bursts. FIG. 2 illustrates the E-UTRA uplink frame structure. A subframe (TTI) 201 comprises two slots 202, 203 and each slot is made up of seven bursts. Six bursts 204 per slot are used for data transmission while one burst 205 is used for the reference signals for the six bursts within the same slot, which implies that the three first slots have to be stored before they can be demodulated when the reference signal burst is received.
The E-UTRA uplink frame structure with only two reference signals per subframe (TTI) limits the frequency hopping flexibility. In E-UTRA, the data bursts are transmitted on the same frequencies as the reference signals. Moreover, the data transmission utilizes the same frequency as the in time closest reference signal. That is, the hopping takes place at the slot boundary and there is only one hop per sub-frame which is illustrated by FIG. 3.
The E-UTRA hopping scheme exploits the available frequency diversity considering the available reference signals but does not fully exploit the possible interference diversity since only one hop per subframe is available. Thus since the interference of FIG. 3 is rather constant over a period of time, only two frequencies are used and a low hopping rate is used, the interference diversity is limited to only two interferers.